Lenght of data

How can i identify the lenght of a data, by example I have a file with the following information:

HMX 01000 01001
RMX 01005 01005
…
…
…
FMX 0005000 5000
FMX 0005008 5010
FMX 0005020 5020

As you can see most of the registers (data) have the same lentgh in the last two columns, but how can I identify the length of this data: 0005000.

I need to read the file and identify that problem.

I will reallly appreciate your help to solve this problem.

Thank You

Best Regards